Color changing indicators are used to provide a visually discernable indication as to the presence of a particular stimulus. For example, ninhydrin (1,2,3-triketo-hydrindene hydrate) is a color changing indicator. Ninhydrin has been recognized as a reagent for detecting amino acids, amines and amino sugars. When reacting with amines, a deep blue or purple product called Ruhemann's Purple forms. The reaction from ninhydrin to the conjugate Ruhemann's Purple is shown below:

A color changing indicator may be directly exposed to a surface containing an associated stimulus. However, with direct exposure to a surface, the color changing indicator that has changed color on the surface may remain on the surface and cause staining damage to the surface.